This invention relates to ink jet head arrangements and, more particularly, to a new and improved ink jet head arrangement having a simple and inexpensive structure.
Conventional ink jet heads, in which ink received from an ink reservoir is ejected selectively through a series of orifices, have been made using thin plates of metal or ceramic material having appropriate passages which are bonded together in adjacent relation in an assembly, as described, for example, in the Roy et al. U.S. Pat. No. 5,087,930 and the Hoisington et al. U.S. Pat. No. 4,835,554. In such arrangements, each chamber or passage in the flowpath leading from the ink inlet to the orifice, through which the ink is ultimately ejected, is provided in one or more of the several plates in the assembly. This requires an array of plates having different thicknesses, each of which must be separately machined to precise dimensions to produce the appropriate chambers and passages, and also requires precise positioning of all of the chambers and passages in the plates. Moreover, the plates must be assembled and bonded together and to a piezoelectric plate in highly precise alignment, and each plate must be flat and free from burrs that would cause voids between adjacent plates. Furthermore, because of differences in the coefficients of thermal expansion between the materials used in the plates, bond stresses are generated by temperature variations which occur in connection with the manufacture and use of the ink jet head which must be overcome.
In hot melt ink jet printheads, which operate at elevated temperatures, the printhead materials must be good conductors of heat so that the printhead will warm up quickly and the temperature gradients during operation will be small. The stresses created when parts of different materials expand differently with changes in temperature is another concern. The prime mover in a printhead is usually a piezoelectric ceramic (PZT) which has a relatively low thermal expansion coefficient. For optimum printhead design, the challenge is to find other materials which are close to this expansion. If the printhead materials cannot be matched, it is desirable to have low-modulus materials to reduce the stresses.
The ink passages in an ink jet printhead are fine features with tight tolerances. To maintain such tight tolerances, the manufacture of the printhead requires low machining forces, small tool deflection and small machining errors, no plastic deformation and no burrs. Moreover, it may be desirable, particularly in development, that the manufacture should be carried out using standard machining methods, such as grinding, milling, drilling and shaping.
In addition, the printhead should be made of materials which are chemically inert and do not change shape over time when loaded or oxidize or interact with organic chemicals found in hotmelt and other inks or with pigments or dyes in the inks.
Heretofore, some plates used in ink jet heads have been photoetched to provide the appropriate chambers and passages, which has the advantage that the plates are generally burr-free and can be made from Kovar metal alloy material, stainless steel and other materials that have appropriate mechanical and thermal expansion characteristics. The materials useful for photoetching, however, have drawbacks when used in connection with ink jet heads from which hot melt ink is ejected since they generally have low thermal conductivity. In addition, the photoetching process has the disadvantage of being a batch process with lotto-lot variations and, moreover, when used in this manner, produces a relatively large quantity of chemical waste.
Futhermore, coventional piezoelectric plates used in ink jet heads are thin, fragile and susceptible to damage during processing. Because of the greater likelihood of damage to larger plates, the maximum size of piezoelectric plates is normally quite small, for example, less than about 100mm, which correspondingly limits the length of an array of orifices through which ink is ejected as a result of the actuation of the piezoelectric plate.
Accordingly, it is an object of the present invention to provide an ink jet head which overcomes the disadvantages of the prior art.
Another object of the invention is to provide an ink jet head having a simple structure which is inexpensive to develop, is convenient to manufacture and is capable of providing high resolution ink jet printing.
These and other objects and advantages of the invention are attained by providing an ink jet head having at least one or more components formed from a carbon member. A preferred carbon component is one in which ink pressure chambers and connecting passages from ink supply lines and to ink jet orifices are formed. In one embodiment, a carbon component is a plate having pressure chambers formed on one side and flowthrough passages to permit continuous ink circulation through the pressure chambers formed on the other side of the plate with connecting passages leading to an orifice plate and to an ink supply extending through the carbon plate. In addition, an orifice plate is affixed to one side of the carbon plate with the orifices aligned with orifice passages in the carbon plate and a piezoelectric plate is affixed to the other side of the carbon plate with actuating electrodes aligned with the pressure chambers to cause the piezoelectric material to be deflected so as to apply pressure to the corresponding pressure chamber and eject a drop of ink through a corresponding orifice in the orifice plate.
In another embodiment, a carbon pressure chamber plate is formed on opposite sides with linear arrays of pressure chambers having ink inlet and outlet passages at opposite ends. Both sides of the carbon plate are covered by corresponding piezoelectric actuation plates and a plurality of such carbon pressure chamber plates are retained in laterally adjacent relation in a carbon collar member with the ink outlet passages therein positioned in alignment with corresponding ink passages extending through a carbon manifold plate. A manifold plate has one side retained against the ends of the plurality of pressure chamber plates and has lateral ink passages formed in the opposite side leading to a line of orifices in an orifice plate mounted on the opposite side.
In accordance with one aspect of the invention, the carbon pressure chamber plate has an ink deaeration passage through which ink is supplied to the inlet passages leading to the pressure chambers and an internally supported, thin-walled tubular member made of air-permeable, ink-impermeable material connected at one end to a source of reduced pressure is inserted into the ink deaeration passage to provide a unitary ink deareating and pressure chamber carbon plate. This arrangement accomplishes the necessary deaeration of ink immediately before it is supplied to the pressure chambers with minimal space requirements and without necessitating recirculation of ink to an ink reservoir.
In connection with the assembly of carbon plate components of the above type in an ink jet head, it has been learned surprisingly that it is not necessary to cement or otherwise physically bond together carbon plate components having communicating a passages or to provide a gasket between them. Because the engaging surfaces of such carbon plate components can be made very smooth and flat and carbon plates are sufficiently rigid to avoid flexing, such plates can be mechanically fastened together by screws or the like without causing ink to flow between the components and, if desired, a filter layer may be interposed between the surfaces of such fastened components.
Because the carbon body can be machined precisely without causing burrs using conventional machining techniques and, since carbon has a low thermal coefficient of expansion, dimensional variations resulting from thermal expansion during machining are minimized on the plate. In addition, the carbon expansion coefficient is especially compatible with the piezoelectric plate which is affixed to it, thereby reducing or eliminating stresses between the plates which might otherwise be produced by temperature variations such as occur when the ink jet head is used with hot melt ink. Moreover, carbon is chemically inert with respect to materials in which it comes in contact in an ink jet head. It does not oxidize nor does it interact with organic chemicals found in hot-melt and other inks or with pigments or dyes used in inks.
According to another aspect of the invention, the piezoelectric plate has actuating electrodes on only one side of the plate and is prepared by a photoetching technique in which a piezoelectric plate coated on one side with electrode material is affixed to the pressure chamber side of the carbon plate with the electrode material-coated side exposed. The exposed side of the plate is coated with a photoresist material and is then exposed to a desired electrode pattern in precise alignment with the pressure chamber pattern in the carbon plate, after which the photoresist is developed, the exposed electrode material is etched away, and the remaining photoresist is removed to produce an electrode pattern conforming exactly in shape and position to the pattern of pressure chambers in the carbon plate. In addition, the electrode pattern thus formed on the piezoelectric plate can include other electrical elements such as a heater to heat ink in the passages in the carbon plate.
In accordance with a further aspect of the invention, the carbon plate is porous, preferably being about 8090% dense, and the porosity and a vacuum source communicating with the surface of the plate can extract dissolved air from ink in the ink passages separated from the porous carbon material by an air-permeable, ink-impermeable layer.
If desired, a page-size carbon plate can be prepared with a row of separate piezoelectric plates affixed to one side of the plate. Moreover, the carbon plate may have orifice passages formed in an edge of the plate rather than in one of the sides of the plate.
Since engineering grade carbon is friable, i.e., microscopic grains are readily broken away from a carbon body, it is easily shaped without producing burrs. As described in xe2x80x9cGraphite Machinery Made Easyxe2x80x9d, EDM Today, September/October 1993 pp. 24ff, the relative softness and lack of ductibility of such carbon allows it to be cut at high feed rates with little distortion and low tool wear. These characteristics permit carbon blocks to be readily formed into components of ink jet heads by conventional or unique machining techniques.
In one example, the formation of an array of closely adjacent pressure chambers for an ink jet head which have a long aspect ratio and require highly precise and uniform channel dimensions, would require prohibitively long machine cycle times using a conventional end mill. In accordance with another aspect of the invention, however, the process for manufacturing a carbon plate component of an ink jet head is greatly simplified by shaping the carbon plate using a series of linear motions against the surface of the carbon plate with a shaping tool having the desired profile. For the pressure chambers of a carbon pressure plate, for example, a tool having a series of closely spaced short teeth is scraped across the surface in several strokes to produce a series of precise channels of the required dimensions. To make a row of small diameter holes of substantial depth in one end of a body, two carbon plates may be shaped in a similar way with matching arrays of grooves having a depth equal to half the diameter of the desired holes and then cemented together with the grooves in alignment. Using certain tool shapes the holes may have a hexagonal shape rather than a circular shape.
Other machining techniques especially useful in shaping carbon bodies are electric discharge machining, which facilitates convenient formation of complex shapes, and laser machining, which can be used effectively for through holes and slots.
According to still another aspect of the invention, improved directionality of ink drop ejection from orifices supplied from non-axial orifice passages is achieved by providing orifice plate orifices having cylindrical outlet nozzle passages, larger diameter cylindrical inlet passages, and a conical intermediate section joining the outlet and inlet passages.